Since the major escalations in the cost of conventional fuels such as oil, coal and natural gas, along with diminishing worldwide reserves of these fuels, increasing attention has been directed to developing alternative energy sources. Solar radiation is an energy source which has long been recognized as a readily available alternative, particularly in certain geographic areas of the world. While solar energy has been demonstrated to be adaptable for some heating purposes, it has been relatively difficult and expensive to achieve adequate performance in other applications in widely varying geographic areas.
Solar energy has been successfully employed to serve an auxiliary heating function such as for swimming pools in warmer climates having a relatively high percentage of sunshine during the year. In most instances the swimming pool water is passed through the solar collectors with the warmed water being returned and recirculated through the swimming pool. These installations typically employ electrically driven pumps for circulating the water. In addition, elaborate control systems are employed to selectively circulate pool water to and from the solar collectors and to shut down the system during under or over heating conditions.
Solar collectors have also been used to heat water in water heater applications of varying sizes and configurations. In a manner similar to the swimming pool applications, the systems characteristically circulate the water to be heated through the solar collectors as effected by an electric pump and numerous related control elements
To the extent efforts may have been made to create a truly efficient system by employing a solar pump in a water heating system, a problem is encountered in the introduction of vaporized fluid to a solar pump during start-up operations. Such start-up operations occur each morning and in instances of extreme overcast where a solar collector becomes inoperative for purposes of converting a working fluid from a liquid state to a gaseous state. During a typical start-up situation there is normally marginally increasing solar energy available so that working fluid is converted to a gaseous state at a relatively slow but perhaps increasing rate. The introduction of minimal quantities of gaseous working fluid through an interconnecting conduit and into a pump constitutes a sufficient volumetric expansion such that the pressure of the working fluid reduces to an extent sufficient to convert all or part of the working fluid to a liquid state such that the pump is incapable of commencing operation to institute the circulation required for the system to perform.
If a valve is employed which requires fluid to reach a sufficient pressure prior to introduction to the pump, the system may nevertheless be incapable of self-starting. This is brought about as a result of the fact that the working fluid upon opening of the valve may be reduced by expansion of the working fluid into the pump and the supply line therefor such that the valve promptly recloses. Upon sufficient build-up of pressure from the solar collector fluid, the valve may again cycle open but fluid introduced to the pump has in the meantime condensed to a liquid state. Thus a cyclic opening and closing of a valve may take place without instituting continuing operation of the pump.
Another problem encountered in the operation of systems for the above applications resides in the use of water as the heat transfer medium passing through the solar collector. In freezing conditions which can take place in many geographic areas at nighttime, the water in the collectors may freeze and seriously damage or destroy the solar collector elements. As a result additional controls and equipment are often incorporated which provide for the drain down of water from the solar collectors when the outdoor temperature drops below a preselected value which could result in the freezing of water contained in a solar collector array. While numerous systems of the above type have been employed in the described applications, elaborate controls and the attendant maintenance have normally characterized these configurations, thereby generally relegating them to auxiliary usage in conjunction with elaborate and expensive water heating and conditioning systems.